NIMA is essential for entry intro mitosis in Aspergillus nidulans. To examine whether there is a NIMA-like pathway in other eukaryotic cell cycles, we expressed NIMA and its dominant negative mutants in two different eukaryotic systems. In Xenopus oocytes, NIMA induced germinal vesicle breakdown without activating Mos, CDC2, or MAP kinase. In HeLa cells, NIMA induced premature mitotic events without activating CDC2, whereas the mutants caused a specific G2 arrest but did not block mutant CDC2-induced premature entry into mitosis. A sequence essential for both these phenotypes was mapped to a region of-lOO amino acids lying just after the catalytic domain of NIMA that shows a significant similarity to protein interaction domains in other proteins. These results provide evidence for the existence of a NIMA-like mitotic pathway in vertebrate cells. The confocal microscopy for this project was carried out at the NCMIR. This work has been published (Lu and Hunter, Cell 81: 413-424, 1995).